As far as well motivated theoretical field theory constructions for
inflation are concerned, there has been considerable progress towards
this goal in recent years, with the construction of several concrete
string inflation models for instance (for reviews, see, e.g.,
Refs. [1]}, [2]} and references therein),
with particular attention given to brane inflation
scenariosThere are two realizations of brane inflation,
namely ultraviolet (UV) and infrared (IR) models, depending on
whether the brane is moving towards or away from the tip of the
throat. In this paper, we consider an UV model (see
Refs. [3]}, [4]} for IR models).. Dirac-Born-Infled (DBI) inflation is one of such string theoretic
motivated model in which the inflaton is interpreted as a modulus
parameter of a D-brane propagating in a warped throat region of an
approximate Calabi-Yau flux
compactification [5]}, [6]}. Hence,
the effective action for DBI inflation contains a special form of the
DBI kinetic term. The DBI kinetic term introduces some novel speed
limits on the inflaton field velocity and helps in keeping it near the
top of potential, even when it is too steep, thus resulting in the
slow-roll phase through a low sound speed, instead of from dynamical
friction due to expansion. A low sound speed, smaller than unity,
allows the inflaton field to have a sub-Planckian evolution, even for
steep potentials, thus circumventing the
\(\eta \) -problem [7]}. Moreover, fluctuations also
propagate with a low sound speed parameter, resulting in both a
smaller tensor-to-scalar ratio and a significant non-Gaussianity,
which can potentially be distinguishable from other
scenarios [5]}, [6]}.
